"Uninstall" reverts any change the extension did to the database, effectively deleting the extension's data.
If you plan to re-install the extension later and want to preserve user data, just remove it with Composer. If you want to remove it definitely and delete all data, disable extension, click "Uninstall", then remove with Composer.
Completely removing the values of the extension settings or permissions from the database will require manual deletion, but it shouldn't hurt to keep those entries.